4 (Woods) Chem-131 Lec Carboxyllic acid 4 Physical Properties Both the carbonyl group and the hydroxyl group in a carboxylic acid are polar and capable of hydrogen bonding. The presence of these two groups allows carboxylic acids to form hydrogen bonded dimers under most conditions: Physical Properties Higher solubility in water than alcohol: three H-bond interactions with water. Solubility decreases more slowly than that of alcohols as the number of carbons increases. 4

5 (Woods) Chem-131 Lec Carboxyllic acid 5 Physical Properties - Acidity Carboxylic acids are weak acids that ionized to form carboxylate ion and hydronium ion in water: The extent of ionization is less than 1-2%. Physical Properties - Acidity Carboxylic acids are weaker acids than the strong acids (HCl, HBr, HI, H 2 SO 4, HNO 3, and HClO 4 ), but stronger acids than phenols, and much stronger than alcohols. Due to the greater electron withdrawing power of the carbonyl group, Compare to a benzene ring, which allows it to spread the negative charge more. 5

6 (Woods) Chem-131 Lec Carboxyllic acid 6 Physical Properties - Acidity Carboxylic acids are acidic enough to turn blue litmus paper red Distinguishes them from alcohols, but not phenols which also turn blue litmus paper red. Sodium hydroxide and sodium bicarbonate react with carboxylic acids to form a carboxylate salt: Physical Properties - Acidity Carboxylic acid in water has a ph 3: only a small percentage of the acid exits as ionized molecules (ph = -log[h 3 O + ]). In alkaline media, the fraction of ionized molecules increases. Physiological Fluids are maintained near neutral ph by the bicarbonate-carbonate buffer system: Blood ph is 7.4 and the ph in most cells ranges from 6.8 to 7.1. At ph = 7, carboxylic acids almost entirely ionized i (carboxylate). 6

7 (Woods) Chem-131 Lec Carboxyllic acid 7 Carboxylate Salt Carboxylate Salt: Product of carboxylic acid and a strong base. The sodium salt of butanoic acid would be called sodium butanoate. Carboxylate Salt Carboxylate salts are ionic and possess much higher boiling and melting points than those of the corresponding carboxylic acids (ionic forces are much stronger than secondary forces). All carboxylate salts are solids at room temperature. Sodium formate: MP 253 C solid at room temperature Formic acid: MP 8 C liquid at room temperature Carboxylate salts are much more soluble than their carboxylic acids. 6+ carbons are slightly soluble or insoluble in water. Sodium stearate (18 carbons) is water soluble. Stearic acid (18 carbons) is insoluble. Increased solubilities in neutral or basic environments because the acids are converted into their carboxylate ions. 7

8 (Woods) Chem-131 Lec Carboxyllic acid 8 Soaps Soaps: usually Na / K salts of long-chain carboxylic acids (fatty acids) Fatty acids are typically unbranched aliphatic chains of 12 to 20 carbon atoms obtained from animal fats and plant oils. The fatty acid carboxylate salts are obtained from fats or oils by a process called Saponification. An example of a soap is the sodium carboxylate salt of stearic acid, CH 3 (CH 2 ) 16 COO - Na +. Nonpolar dirt (eg. grease) does not dissolve in water because it is hydrophobic. Amphipathic: Hydrophilic (water-loving) and Hydrophobic (water-hating) parts. Soaps The hydrophobic tail of a soap molecule has an affinity for the greasy dirt; the hydrophilic head, for water: The requirements for a good soap are an ionic group as the hydrophilic head and a long hydrocarbon group as the hydrophobic tail. Stearic acid (C 18 ): Not a soap, because no ionic head group. Its carboxylate salt is a good soap, however. Sodium butyrate, CH 3 (CH 2 ) 2 COO - Na +, Not a good soap because its hydrocarbon tail is not long enough (not sufficiently hydrophobic). 8

9 (Woods) Chem-131 Lec Carboxyllic acid 9 Esters Ester is formed when Acid is heated with an alcohol (R = aliphatic) or a phenol (R = aromatic) in the presence of a strong acid catalyst. This process is called Esterification. Esterification is a reversible reaction. Removal of water shifts the equilibrium to the right. Esters Esters in organisms are synthesized by reactions catalyzed by enzymes. Esters can also be formed from the reaction of an alcohol (or phenol) with a carboxylic acid anhydride or halide, rather than a carboxylic acid. Esters of carboxylic acids have pleasant odors and flavors: Pineapple ethyl butanoate Raspberry isobutyl methanoate Banana 3-methyl-butyl ethanoate Various esters are used to create the fragrances of perfumes. 9

10 (Woods) Chem-131 Lec Carboxyllic acid 10 Aspirin 2-hydroxybenzoic acid (salicylic acid) + acetic anhydride Aspirin (acetylsalicylic acid) an ester [2-Acetyloxybenzoic acid] Nomenclature of Esters Esters are derived from the names of the alkoxy or aryloxy (alcohol or phenol) part and the acyl (carboxylic acid) part of the compound. 10

11 (Woods) Chem-131 Lec Carboxyllic acid 11 Nomenclature of Esters Much lower boiling and melting points than carboxylic acids. No H-bond to itself. Propanoic acid Methyl acetate Physical Properties of Esters 141 C 57 C Less soluble in water than carboxylic acids. Not as many H-bonds to water. Lower melting and boiling points than aldehydes. Methyl acetate Butanal 57 C 76 C Same water solubilities as aldehydes and ketones. All three hydrogen bond to water equally well. 11

12 (Woods) Chem-131 Lec Carboxyllic acid 12 Hydrolysis of Esters Acidic hydrolysis of an ester is the reverse of the acid catalyzed formation of an ester. Both are Equilibrium Reactions. The equilibrium can be shifted to favor hydrolysis by carrying out the reaction with a large excess of water. Basic hydrolysis with a strong base (NaOH or KOH). Not Reversible. Saponification: Manufacture soaps from animal fats and vegetable oils. 12

13 (Woods) Chem-131 Lec Carboxyllic acid 13 Carboxylic Acid Anhydrides & Halides Anhydride: A compound that is formed by dehydration. Acid anhydrides are highly reactive with water. Not found in biological systems. Carboxylic acid anhydrides: Change the word acid to anhydride. Carboxylic Acid Anhydrides & Halides Acid anhydrides react with alcohols (or phenols) to form esters: Ester synthesis from anhydrides or acid halides is more efficient than from the carboxylic acid itself. 13

15 (Woods) Chem-131 Lec Carboxyllic acid 15 Phosphoric Acid Anhydrides Di & Triphosphoric acids are anhydrides as well as acids: Esterification to form Phosphoric acid esters or Phosphate esters. Named in the same way as carboxylic esters: The name of the alcohol (or phenol) is followed by the name of the acid with its ending changed from ic to ate. 15

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